How to change the key of a chord progression

Take four chords in C major: C – G – Am – F. Now play them all two semitones higher (a semitone is the step between two neighbouring keys, a tone is two semitones). C becomes D, G becomes A, Am becomes Bm, F becomes G. That is transposing: moving a whole progression to another key.

Listen to both versions. The progression is recognisably the same, only higher. On the keyboard, though, the shape changes: two black keys appear, F♯ and C♯. They are not a mistake; D major needs them.

To transpose without counting every key, number the notes of the scale from 1 to 7, starting on the tonic. Each one carries a chord built from the scale, called a degree and written in Roman numerals: capitals for major chords, lower case for minor ones, and a small circle for the diminished chord. In C major:

I C · ii Dm · iii Em · IV F · V G · vi Am · vii° Bdim

Our progression is therefore I – V – vi – IV. That description holds in every major key: the I chord is always major, the vi chord always minor. Once you know the degrees, you only need the scale of the new key to find the chords.

Pick a key in the menu. The degrees stay put, the chords and their notes change. Notice how the notes are spelled: each key writes its chords with its own scale, so D major shows F♯ in D and in Bm, never G♭, and F major shows B♭ rather than A♯.

The question under the keyboard checks the same idea: find the chord on a degree, in a key you have not played yet.

The method fits in two steps. First, find the degree of each chord in the original key. Then, in the new key, take the scale and build the same degrees. Checking that each chord moved by the same distance is a good way to catch a mistake.

The most common reason is a voice: a song that is too high or too low for a singer moves down or up until it fits. Another is the hand: some keys lie more comfortably under the fingers, and a progression you know in C becomes a good way to discover G or F major, each with a single black key (F♯ and B♭).

Transposing is also a fine exercise in itself: play the same four chords in a new key each week, and the degrees end up sounding familiar wherever they are.